SetProperty missing in FreshBasePageModel

[EmilAlipiev]

Hi,

maybe small thing but it will be nice to have this included because of upgrading existing projects. I can send a PR for this

 protected bool SetProperty<T>(ref T backingStore, T value,
            [CallerMemberName] string propertyName = "",
            Action onChanged = null)
        {
            if (EqualityComparer<T>.Default.Equals(backingStore, value))
                return false;

            backingStore = value;
            onChanged?.Invoke();
            RaisePropertyChanged(propertyName);
            return true;
        }